Organic electroluminescent devices such as organic light emitting diodes (OLEDs) are presently of great interest due to their potential application in high efficiency, flat panel displays. OLEDs are described in U.S. Pat. Nos. 6,013,384 and 6,579,029 each of which are hereby incorporated by reference as if set forth in their entireties. OLEDs were originally demonstrated as the light emitting component in passively addressed display products. Passive matrix displays demonstrate the feasibility of OLEDs in many applications, but encounter a fundamental barrier as the display, size and pixel density of the display device increases. Because the luminescent output of an OLED is proportional to the charge injected through the device, the current densities required to operate passively addressed displays rapidly rise as the time available to drive each pixel decreases with increasing display resolution. These higher current densities cause large voltage drops in the indium tin oxide (ITO) or similar interconnect lines of the passive matrix array, push the OLED operation to higher voltages and create display driver issues that are not easily resolved.
In response to such issues, active matrix drive schemes have been developed and active matrix OLEDs (AMOLEDs) are therefore of particular interest. Active matrix technology is a method of sending charges to pixels of a light emitting display. A common example of an active matrix display is a TFT, thin film transistor, commonly formed using polysilicon technology. Whereas a passive matrix display uses a simple conductive grid to deliver current to the pixels of the matrix, an active matrix display uses a grid of transistors with the ability to hold a charge for a limited period of time. Because of the switching action of transistors, only the desired pixel receives a charge, improving image quality over a passive matrix. Furthermore, because of the thin film transistor's ability to hold a charge, the pixel remains active until the next refresh. A goal in AMOLED display technology is to generate a constant current source at each pixel using such thin film transistors. Each pixel is programmed to provide a constant current during the entire frame time, eliminating the high currents encountered in the passive matrix approach. However, unlike LCD-thin film transistor technology, for example, the lighting control of OLEDs is self generated and all of the current supplied to the TFTs to enable the TFTs to drive the OLEDs, flows along a thin layer of conductive material formed on the surface of the substrate and which forms a power transmission line and various interconnect lines within the array. For small arrays such as a 2 square inch array, the current required in the power transmission line that delivers current to the array, may be at manageably low levels such as 300 milliamps. For large displays such as a 30-inch display, the total current needed may increase to 5-10 amps which affects the dependability of the thin power transmission lines conventionally formed on the substrate/array surface. Due to the resistance of the very thin material typically used as conductive interconnects such as ITO (indium tin oxide) aluminum or copper, such a current may result in a potential difference of as much as 27V across the power transmission lines and interconnect lines. Such a potential voltage may cause the interconnect material to burn out, resulting in opens that render the display device inoperable.
Another shortcoming is that the resistance of the thin interconnect materials typically used as power transmission lines and interconnect lines, may produce variable power across the matrix. The lighting intensity of the pixels and therefore can result in a non-uniformly illuminated display.
It would therefore be desirable to provide a display device including an active matrix organic light emitting diode and including a power transmission line capable of accommodating large currents associated with active light emitting devices manufactured to include large dimensions.